M8 



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Technical Paper 188 



P 467 
H8 
opy 1 



DEPARTMENT OF THE INTERIOR 

. ALBERT B. FAHL, Secretary 

BUREAU OF MINES 

H. FOSTER BAIN, Director 



CORROSION UNDER OIL FILMS, WITH SPECIAL 

REFERENCE TO THE CAUSE AND PREVENTION OF 

THE AFTER-CORROSION OF FIREARMS 




BY 



WILBERT J. HUFF 




/I 



a 



_ -2 b ^ ^^ 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1922 






The Bureau of Mines, in carrying out one of the provisions of its organic 
act — to disseminate information concerning investigations made — prints a 
limited free edition of each of its publications. 

When this edition is exhausted copies may be obtained at cost price only 
through the Superintendent of Documents, Government Printing Office, Wash- 
ington, D. C. 

The Superintendent of Documents is not an official of the Bureau of Mines,. 
His is an entirely separate office and he should be addressed : 



Superintendent of Documents, 

Oovernment Printing Office, 
Washington, D. 



C. 



The general law under which publications are distributed prohibits the giv- 
ing of more than one copy of a publication to one person. The price of this 
publication is 5 cents. 

Persons desiring for lecture purposes the use, free of charge, of lantern slides 
of the illustrations in this publication should make r<^quest of the Director of 
the Bureau of Mines, Washington, D. G. 



First edition. April, 1922. 



II 



LIBftARY OF CONGRESS \ 

RECEIVED 

AUG 9 1922 

DOCUMENTS D!V.JI >. 



<^ 



A^'^ 



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CONTENTS. 



Page. 

Introduction 1 

Historical review 2 

Theories of after-corrosion 2 

Corrosive powder residues 2 

The powder-gas occlusion theory 3 

Noncorrosive priming compositions 4 

The metal-fouling theory 5 

Empirical observations of practical riflemen 5 

Experiments made 5 

Rifles, ammunition, oil, and equipment 5 

Preliminary experiments 6 

Firing tests at known humidities 6 

Cupro-nickel fouling not a cause of after-corrosion 8 

No evidence of gas diffusion 9 

Acid residues on the bore surface '. 10 

Potassium chloride corrosive fouling 11 

Possible objections to the potassium chloride theory 11 

Failure of mechanical cleaning processes 12 

Sw^eating and the occurrence and recurrence of corrosion over long 

periods of time 1 12 

After-corrosion at humidities below saturation 13 

The mechanism of tifter-corrosion • 14 

Oxygen a contributing factor 15 

►Severity of after-corrosion and its continuance under conditions such 

that unoiled steel surfaces are not corroded 15 

After-corrosion in arid regions 15 

Possible absence of after-corrosion in high atmospheric humidities- 15 

Prevention of after-corrosion ^ 113 

Exclusion of water vapor 16 

Solution and removal of potassium chloride 16 

Nonaqueous " nitrosolvents " and " gun oils " 17 

Oil-water emulsion 19 

A test for useful solvents . 21 

Addition of an inhibitor of corrosion 22 

Black powder residues 22 

Noncorrosive priming compounds 22 

After-corrosion from low-pressure nitrocellulose powders ^ 24 

The general problem of corrosion under oil films 24 

Summary 25 

Acknowledgments 26 

Publications on the coiTosion of iron and steel 26 

ITI 



IV COITTEXTS. 

TABLES. 



Page. 
Table 1. Relations between corrosion, humidity, and length of exposure 7 

2. Relative corrosion in water, in ammonium oleate plus water, and 

in an oil-water emulsion containing ammonium oleate 20 

3. Relative corrosion in air after immersion in various media 20 



ILLUSTRATIONS. 



Plate I. Swabs from rifle barrels (see Table 1, G, D, and E) 6 

II. Successive first swabs from one of the rifles of Table 1, C 7 

III. Swabs from one of the rifles of Table 1, D 8 

IV. A, Section of corroded rifle bore ; B, Photomicrograph showing tool 

wounds and fissures on the bore surface of a new barrel 9 



CORROSION UNDER OIL FILMS, WITH SPECIAL REFERENCE TO THE 
CAUSE AND PREVENTION OF THE AFTER-CORROSION OF FIREARMS. 



By Wilbert J. Huff. 



INTUODTJCTION. 

Toward the end of the World War the Bureau of Mines was re- 
quested to investigate the causes of after-corrosion upon the bore 
surfaces of the infantry service rifle, with the ultimate purpose of 
developing* some simple procedure for eliminating this serious men- 
ace. The bureau was especially fitted for conducting such an in- 
vestigation because of its previous w^ork on the preservation and 
utilization of our metal resources, and because of the experience of 
certain of its members in explosives and oils. Experiments had 
hardly begam when the armistice was signed, but as the problem 
is important because of its applications both in w^ar and in peace, 
continuance of work on it was deemed advisable. The Ordnance 
Department ^ of the Army says that probably more rifles are ruined 
by improper preparation for storage than by any other cause; and 
the problem, though primarily military, touches the interest of 
every owner of a firearm. 

The importance of this study is not, however, limited to the users 
of firearms. The fundamental problem proved to be corrosion under 
oil films; this differentiates after-corrosion sharply from the ordi- 
nary corrosion of clean iron and steel surfaces. It will be shown 
that this after-corrosion is closely allied to a number of other 
general problems, such as the corrosion under oil of bright steel 
parts after handling in manufacturing operations, and to the corro- 
sion under oil experienced near the ocean. 

Because of the wide application of the results of the study, it 
was deemed advisable to publish them in detail to serve both as a 
guide for proper protective measures and as a basis for further 
research by the investigators of metals, of oils, or of explosives who 
might otherwise be obliged to duplicate some of the work described 
here. 

^ Ordnance Department, Description and rules for the management of the United States 
rifle, caJiber .30, model of 1917, 1918, 45 pp. 

1 



2 COKROSION UNDER OIL FILMS. 

HISTORICAL REVIEW. 
THEORIES or AFTER-CORROSION. 

CORROSIVE POWDER RESIDUES. 

Generally, corrosion under an oil film has been attributed to acid 
])roducts of explosion, such as acid nitrocellulose residues, or acids 
from the primer, or to diffusing acid gases, although other explana- 
tions have been advanced. 

Probably those theories that entail the presence of nitric residues 
on the bore surface preponderate. Thus, Guttman^ states, "Most 
smokeless powders do not leave any residue worth speaking of ^and 
the barrels are perfectly clean after a shot, but after using they 
must be carefully cleaned, because traces of nitric compounds always 
remain, which, when combined with the action of the moisture in the 
air, will attack the barrel and rifling." 

The following selections will show how widely this seemingly 
authoritative opinion is disseminated. The Beck ^ patents state : 

As is well known, there are produced on explosion in firearms, especially 
in those firing so-called "smokeless powder" * * * residues of nitro- 
conipou-ixls. These resiihies are forced violently by the pressure of the gas 
liberated into the rifling, or in the case of the barrels of shotguns into the 
roughness or irregularities of same, and finally under the action of atmospheric 
moisture cause rusting or corrosion of the firearms. 

Wild ^ sought " to produce a nonaqueous lubricant which shall 
neutralize the acid residue left after firing smokeless powders, and 
so prevent rusting or corrosion." 

Klever ^ has patented a process for preparing emulsifiable oils 
whicli, he says, " are capable of dissolving or removing the destruc- 
tive residues which remain in the bores of firearms when fired with 
nitric powders." 

Braun and the Saponia-Werke Ferdinand Boehm ^ patented a mix- 
ture whose active agent is sodium hydroxide, which, according to the 

-Guttman, O.. The manufacture of explosives. London, Vol. II, 1S05, p. 274. 

^ Beck, August, Metal preserving and cleansing compound, U. S. Pat. 719,074, Janu- 
ary 27, 1903 ; Improvements in preparations for protecting the barrels of firearms from 
rust or corrosion and for cleaning the same, British Pat. 15,078, 1901 ; Rostschutz-und 
Reingungsmittel fiir Laufe von Feuerwaffen, Austrian Pat. 9,766, November 10. 1002. 

* Wild, Joseph G., Alkaline lubricant for oiling guns, U. S. Pat. 768, Soo, August 30, 
1904. 

5 Klever, Fi-iodrich W., Manufacture of lubricating and anticorrosivo oils. U. S. Pat. 
919, .SS4, April 27, 1909; Improvements in and relating to the manufacture of emulsifiable 
compositions from hydrocarbon oils, British Pat. 27.254, 1905 ; Verfahren zur Ilorstellung 
eines Ilostschutz-und Schmiern\ittels, ensbesondere zur Be.soitungung der sogenaunten 
Nachscbliige aus Laufeu in Schusswaffen, German Pat. 174.906, Klasse 23c. Oruppo i, 1905. 

''• Braun, Wilholm, Manufacture of a cleaning and protecting material for motals, U. S. 
I'at. S()2,305, August 0, 1907 ; Verfahren zur Ilerstellung eines Heinigungsmlttels fiir 
Gevvelirlaufe und andere Metallgegeristando, Austrian Pat. 32,043, April 25. 1908. 



HISTORICAL REVIEW. S 

patent claims '' combines with the acid residues due to the explosion 
of the powder." 

The Ordnance Department of the United States Army seemingly 
supports the above explanation when it says : ^ 

Powder fouling, because of its acid reaction, is highly corrosive ; that is, it 
will induce rust and must be removed. Metal fouling of itself is inactive but 
may cover powder fouling and prevent the action of cleansing agents until 
removed. * * * 

This very plausible explanation for after-corrosion is strengthened 
by Marshall,^ an authority on explosives, when he says " — black 
powder * * * has been able to hold its own in certain fields in 
consequence of its advantages ; * * « and the noncorrosive na- 
ture of the residue which it leaves in the gun." 

THE POWDER-GAS OCCLUSION THEORY. 

A slightly different explanation has been advanced by Pasdach ^ 
and by others. According to this theory, at least some of the gaseous 
products from the explosion of the smokeless powder are acid. These 
are driven into the bore surface while it is heated by the explosion 
and the friction of the bullet. On cooling, these gases are then 
supposed to diffuse slowly to the surface where, in combination with 
moisture, they cause corrosion for a long time in spite of continued 
cleaning. 

Pauling ^° concurs with this theory, which seems to be corroborated 
by the experience of many marksmen who find that a bore from which 
all visible fouling has been swabbed by an ordinary mineral or ani- 
mal oil, slowly corrodes under such an oil. 

One modern treatise on rifles and their ammunition says: 

In these days of modern smokeless powder the chemical action of the de- 
posit left in the barrel after combustion causes the immediate appearance of 
rust in a most violent form. The corrosive action begins immediately after 
the barrel has been discharged, and, owing to the pressure set up in the bar- 
rel, some of the acid products of combustion may be said to be forced into the 
skin beneath the surface, making it impossible to free the bore entirely from 
this chemical fouling by the ordinary process of cleaning.^! 

' Ordnance Department, -^^ork cited, p. 4.3. 

^ Marshall, Arthur, Explosives, their manufacture, properties, tests, and Mstorj'. Lon- 
don, 1915 ed., p. 61. 

» Pasdach, — , Removal of the after-effects on weapons using nitrate powders by the use 
■of Klever's Ballistol oil, Kriegstechnische Ztschr. 9, 1906. (From a translation by H. E. 
Fleischner.) 

^•^ Pauling, Curt, Precede de preparation d'un produit antirouille (Process for preparing 
an anticorrosive product). French Pat. 409,569, 1910. 

" Ommundsen, H. and Robinson, E. H., Rifles and ammunition and rifle shooting. New 
York, 1915, pp. 282-283. 



4 COKROSION UNDER OIL FILMS. 

NOXCORROSIVE PRIIVtlXG COMPOSITIONS. 

Patent literature records many attempts to prevent or diminish 
corrosion by modifying the primer composition. The theories in- 
volved are in many respects contradictory. 

The first patent was granted to Zeighler in 1900.^^ The composi- 
tion contained a mixture of barium nitrate and barium carbonate 
instead of potassium chlorate, thus eliminating chlorine, which, ac- 
cording to the patent claims, is a cause of the rusting. The barium 
carbonate was said to combine with the acid gases produced by the 
explosion, and to prevent the decomposition of the fulminate by the 
other constituents. 

Four years later the Westfalisch-Anhaltische Sprengstoff-Actien- 
Gesellschaft patented mixtures which contained a chromate, claim- 
ing that the products from the decomposition of this formed a pro- 
tective coating on the barrel and so prevented rusting. Part or 
all of the chlorate Avas displaced.^^ 

Lang,^* believing that the elimination of oxygen diminished the 
production of acids and so prevented rusting, used a mixture con- 
taining only mercury fulminate, metal poAvder. and sulphur. 

Eley Brothers and Ernest Goodwin ^^ say that corrosion is much 
diminished when there are used only cartridges primed with a 
chlorate-free mixture containing barium peroxide and trinitro- 
toluene. 

Meyer patented the use of barium nitrate, alone and mixed Avith 
lead peroxide. To increase the force of the resulting primer he 
added potassium picrate.^^ 

Claessen and the Rheinisch-Westfalische Sprengstoff-Actien-Ge- 
sellschaft have patented nitrogen sulphide priming compositions said 
to be rustless. The French patent mentions potassium chlorate as 
a possible ingredient, the German does not.^^ 

12 Zeighler, H., German Pat. 122,389, Austrian Pat. 7074. (From Escales, Richard, and 
Stettbacher, Alfred, Initialexplosivstoff. Leipzig, 1917, pp. 7, 331-333, 369-370.) 

^3 Westfalisch-Anhaltische Sprengstoff-Act.-Ges., A^erfahren zur Herstellung von Knall- 
quecksilberzundsatzen (Process for preparing priming compositions containing mercury 
fulminate), German Pat. 176,719, Klasse 78e, Gruppe 2. 1904: Proc^d4 pour la fabrica- 
tion de charge d'amorce, French Pat. 348,721, 1905. 

1* Lang, Albert, Verfahren zur Herstellung von metallhaltigen Knallquecksilberzundsat- 
zen (Process for preparing mercury fulminate priming compositions which contain metal). 
German Pat. 209,812, Klasse 78e, Gruppe 2, 1908. 

''^ Goodwin, Ernest, Detonating composition, U. S. Pat. 1,029,287, June 11, 1912: Eley 
Bros. (Ltd.), arid Ernest Goodwin, Improvements in detonating compositions for use in 
rim-fire cartridges, percussion caps, detonators, and the like, British Pat. 2682. 1911. 

>« Meyer, Wilhelm, Improvements in detonating comiwunds for use in rim-fire cartridges 
iind pcu'cussion caps, British Pat. 21,337, 1911 ; Improvements in and relating to the 
inannfacture of priaiing compounds for ammunition, British I*at. 23.49;>. 1911 (addition to 
21,:!37. IfUl). 

'■^ Khcinisch-Westfalische Sprengstoff-Aot.-G(>s. in Coin. A'erfahren zur Herstellung von 
Zundsatzen fur Gewehrzundhutclien uud Zundhuteheu kleinerer Handfeuerwaffon (Process 
for preparing priming compositions for rifle primers and primers for small arms), Ger- 
man Pat. 277,560, Klasse 78e, Gruppe 2, 1913 ; Classen, Conrad. I'roct^de pour la preparar- 
tion de composition fusantes on fulminantes (Process for the preparation of burning or 
detonating compositions), French Pat. 455,3G1>, 1913. 



HISTORICAL REVIEW. 5 

Mertens ^^ held that the acids formed by the decomposition of hal- 
ogen compounds together with air and moisture, were the cause of 
after-corrosion. As a substitute for the chlorate he recommended 
barium nitrate, with an increased percentage of mercury fulminate. 
He also regarded corrosion from smokeless powder possible. 

Whelen ^^ in his treatise on '* The American rifle," states : 

The fouling of smokeless powder of itself is seldom harmful to steel * * * 
but the fouling of the primer is extremely acid and at once gives to the entire 
fouling a very acid character. 

Marshall holds that the chlorate forms chloride which causes the 
barrel to rust.-'- 

THE METAL-FOULIXG THEORY. 

The prominent place among modern theories of corrosion accorded 
to the electrochemical theory, coupled with the anticorrosive action 
of solutions that remove metal fouling, has suggested still another 
possible cause — the potential difference at the contact between the 
steel and the foreign alloy. 

EMPIRICAL OBSERVATIONS OP PRACTICAL RIFLEMEN. 

A compilation of the observations of a number of practical riflemen 
only added to the confusion of these conflicting opinions. Some had 
never encountered after-corrosion under oil. Others invariably en- 
countered it save when cupro-nickel was removed. Some thouofht 
after-corrosion could be eliminated by rubbing the nose of the bullet 
with a heavy lubricant. Many testified to the '' leaking " effects sub- 
sequent to cleaning. 

EXPERIMENTS MADE. 

RIFLES, AMMUNITION, OIL, AND EQUIPMENT. 

Four United States Infantry rifles, two of the model of 1903 and 
two of the model of 1917, were used in the experiments made; to 
duplicate service conditions the rifles were used as received without 



IS Mertens, Otto, Rostlreie Zundsatze (Noncorrosive priming compositions), Ztsclir. 
ges. Schiess-und Sprengstoffw., vol. 9, 1914, pp. 70-1. 

18 Whelen, Townsend, Tlie American rifle. New York, 1918, p. 599. 

•-"Marshall, Arthur, Explosives, their manufacture, properties, tests, and history, Lou- 
don, 1915, p. 426 ; this paper, p. 22 ; Munroe, C. E., in a private communication has called 
the writer's attention to a reference dated 1825. which mentions the corrosive action of 
potassium chloride deposited by powders containing potassium chlorate ; Aubert, Pelissier, 
and Gay-Lussac, Rapport sur les poudres fulminantes poiivant servir d'amorces aus armes 
a feu, Ann. Chim. et Phys., 42 ser. 2, pp. 5-25 (extract from Archives de la direction des 
poudres et salpetres, 1825). 

88924—22 2 



6 COEROSION UNDER OIL FILMS. 

special treatment. Standard service cartridges primed with typical 

fulminate, sulphocyanide, and sulphur compositions were used, these 
cartridges coming from Frankford arsenal, the Remington-TJ. M. C. 
Co., the Winchester Repeating Arms Co., and other sources. No 
qualitative differences were ever noted in the corrosive effects of 

ammunition from the different sources. 

The oil employed was a widely known brand of light oil having 

the following composition : 

Analysis of oil uscd.'^ 

Per cent. 

Animal oil (possibly prime lard oil) 52.50 

Mineral oil (34° B. neutral oil, much used as a spindle oil) 47.50 

Free acid (probably oleic acid in tlie animal oil) 1.32 

In this paper this oil is denominated oil A. In the tests it gave 
excellent protection against atmospheric corrosion and practically 
none against after-corrosion. These properties rendered it very 
useful. 

To control the humidities to which the fired rifles were exposed, 
two wooden cases lined with galvanized iron and having snug-fitting 
covers were designed as humidors. Each could hold comfortably 
four fully assembled rifles, which were placed so that the barrels lay 
horizontally. The bottom of each case was covered with glass pho- 
tographic trays to serve as containers for the salt or solutions used 
to control the humidity. 

PRELIMINARY EXPERIMENTS. 

Before the writer began active experimenting a number of pre- 
liminary firing tests were made, using many hundred rounds of 
service ammunition. Then the rifles were carefully swabbed out 
with cloth and oil A. They were exposed to an indoor atmosphere 
during several winter months but no after-corrosion appeared. 

FIRING TESTS AT KNOWN HUMIDITIES. 

The first tests made by the writer were carried out as follows: The 
rifles without previous treatment were fired until 40 rounds had been 
expended in each. Each bore was then swabbed with cloth and oil 
A until it appeared bright and the SAvabs were no longer discolored. 
A generous coating of oil was allowed to remain on the bore. Over 
the trays in one humidor was spread anhydrous calcium chloride 
to give a humidity of approximately zero, and in the trays of the 
other was poured a 44 per cent solution of sulphuric acid to give a 

-> Analyses by N. A. C. Smitli, of tlio petroleum section of the rittshnrsrh station. Bureau 
of Mines. 



iUREAU OF M I NES 



TECHN ICAL PAPER 





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SUCCESSIVE FIRST SWABS FROM ONE OF THE RIFLES OF TABLE 1. C: 
a, FIRST DAY; /;, SECOND DAY; c, SEVENTH DAY; d. NINTH DAY; 
e, ELEVENTH DAY;/, FOURTEENTH DAY. 



EXPEKIMENTS MADE. 

hnmidky of approximately 50 per cent. The rifles were then hung 
nice two in each humidor. Xo after-corrosion appeared >n any 
m ouU thev were allowed to remain under these conditions for 22 

divs aTshown in Table 1, A and B. The rifles were then employed 
\f r^^tT^f Table 1 C and D. To obtain the humidity of approsi- 

maSly Too ;erctnt^.tke calcium chloride was removed and the trays 

' Tab]lt rsirrSiat the corrosion experienced was after-corro 
sion whick e -idently requires a humidity greater than approximately 
50 percent Plate I shows this clearly. The swabs photographed are 

ho's of June 9, Table 1, C, D, and E. Plate II shows tl- succes-e 
first swabs from one of the rifles of Table 1. C. "First day etc 
Signate the day on which the swab was obtained, and .^reckoned 
from the date on which the sodium carbonate was used. The corre 

ponding swabs from the other rifles were similar m appearance to 
Jhose shtwn. Plate III shows the appearance of the swabs from one 
of the rifles of Table 1, D. " Third day, 50 per c«nt humidity, etc., 
here show the relation between humidity and length of exposure. 



Table 1. — Relations lieUceen 



corrosion, humidity, and iength of exposure. 



A. Humidity, zero. 



Date. 



May 14 

May 28 
June 5 



B. Humidity ap- May 14 
proximately 50 
per cent. I ^ 

^ I May 23 



June 5 



Rifle. 



C. Humidity 100 
per cent 

throughout. 



June ti 

June 7 
June 9 



Humidity, per cent. 



Remarks. 



Zero. 

Zero. 
Zero. 



Approximately 50. . 

Approximately 50. . 
Approximately 50. . 



C Atmospheric. 

D : 



C 100 . 

C ' 100 
D 



Days after 
washing: with 
sodium carbonate 
solution- 
First i June 10 



Second \ June 11 

Seventh...- June 16 

Ninth i June 18 

Eleventh...! June 20 



r 


100 


D 

c 


100. 


D 




c 


100 


D 




c 


100 


D 


1 


C 
D 


i 100 



40 rounds fired in each rifle: bores then 
swabbed with oil A, and exposed over cal- 
cium chloride. 

No corrosion. 

No corrosion. 

(Rifles used without change in the next firing 

! JTo£^L fired in each rifle: ^ ores then 
i swabbed ^v-ith oil A, and exposed over 44 
I per cent sulphuric acid. 
! No corrosion. 

No corrosion. 

(Rifles used without change in the next firing 

lOO^'JoiSlds fired in each rifle: bores then 
swabbed with oil A. 

Rifles placed in the humidor containing pure 

Boris" of both rifles heavily ^ corroded, all 

other metal parts bright and unattached. 

Cold samrated sodium carbonate solution 

''pumpldTp and do^^•n through each a few 

times, followed by water and dr> patches. 

Finaliy rifles oiled with oil A and reexposed. 



Trace of corrosion found in each bore was 
wiped off with cloth swab and oil A. 
Rifles then reexposed. 

No corrosion. 



No corrosion. 
No corrosion. 
No corrosion 



8 COREOSIOIvr UNDER OIL FILMS. 

Table 1. — Relations hetween corrosion, hurnidity, and length of exposure 

Continued. 



C. Humidity 100 
per cent through- 
out—Continued. 
Days after 
washing with 
sodium carbonate 
solution— Contd. 

Fourteenth 



D. Humidities 50J 
per cent and 100 
per cent. 



Days after ex- 
posure — 
Second 



Third . 

Ninth. 



Days after trans- 
fer to 100 per 
cent liumidi- 
ty- 
Second 



Fourth.. 
Seventh. 



1. Humidity 100 
per cent— Ivfew 
barrels, never 
fired. 



Date. 



June 23 

June G 
June 7 

June 9 
June 10 
June IG 

June 18 

June 20 
i June 23 



June 7 
June 9 
June 10 
June IG 
June 18 
June 20 
June 23 



Rifle. 



Humidity, per cent. 



100. 



Atmospheric . 



50. 



50. 



100. 
100. 



100. 
100. 
100. 
100. 
100. 
100. 
100. 



Remarks. 



No corrosion. 

On June 23 l^oth rifles treated with metal- 
fouling solulions,a a deep blue showed thera 
presence of cupro-rickel. 

100 rounds flred in each: bores then swabbed 
with oil A. 

Rifles' placed in humidor containing 44 per 
cent sulphuric acid. 



No corrosion. 

No corrosion. 

No corrosion. Both rifles transferred to hu- 
midor containing water. 



Heavy corrosion in bores of both: all other 
parts bright. Bores swabbed clean with 
cloth and oil and reexposed. 

Continued corrosion in bores of both rifles, 
less than that noted on .Tune 18. 

No corrosion. On June 23 both rifles cleaned 
with metal-fouling solution, which turned 
a very dark blue, showing cupro-nickel 
fouling in each. 

Both barrels coated with oil .\ and exposed. 

No corrosion. 

No corrosion. 

No corrosion. 

No corrosion. 

No corrosion. 

No corrosion. 



a Ccmpcsition of metal-fouling solution: 

Ammonium persulphate 28 grams (1 ounce). 

Ammonium carbonate 13 grams (200 grains). 

Ammonium hydroxide (28 per cent) 177 c. c. ((> ounces). 

Water 118 c. c. (4 ounces). 

(Ordnance Department, description and rules for the management of the United States rifle; calibe 
0..30, model of 1917.) 

CUPRO-NICKEL FOULING NOT A CAUSE OF AFTER-CORROSION. 



The plates show that the fired rifles gave some color to all of the 
patches. " No corrosion," indicates that this color was a light green. 
Tested with potassium ferrocyanide and acetic acid, this green gave 
the raahogany brown of copper ferrocyanide. The detached corner 
of the swab " Ninth day," Plate II, was tested in this man- 
ner. The metal-fouling solution used on June 23 likewise showed 
that cupro-nickel fouling was present throughout the tests. As 
rifles C and D, Table 1, C, had ceased to corrode after Juno 10, 
there is no evident connection between cupro-nickel fouling and 
after-corrosion. 



BUREAU OF M I NES 



TECHNICAL PAPER 188 PLATE 









d 





S\Nk^'~> FROM ONE OF THE RIFLES IN TABLE 1, D, RIFLES TRANSFERRED 
TO 100 PER CENT HUMIDITY ON THE NINTH DAY: a, THIRD DAY, 50 
PER CENT HUMIDITY; 6, NINTH DAY, 50 PER CENT HUMIDITY; c, SECOND 
DAY, 100 PER CENT HUMIDITY; d, FOURTH DAY, 100 PER CENT HUMID- 
ITY: t, SEVENTH DAY, 100 PER CENT HUMIDITY. 



BUREAU OF M INES 



TECHNICAL PAPER 188 PLATE IV 





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A. SECTION OF CORRODED RIFLE BORE. 




Ji. PHOTOMICROGRAPH SHOWING TOOL WOUNDS AND FISSURES ON 
THE BORE SURFACE OF A NEW BARREL. 



EXPERIMENTS MADE. 9 

To establish this point. tAvo new barrels, free from grease, were 
fouled by driving through them, by mechanical means, bullets re- 
moved from service cartridges. With each succeeding bullet the driv- 
ing grew more difficult. In one barrel the jacket of the fifth bullet 
stuck; further driving carried out most of the lead core, leaving the 
cupro-nickel jacket about halfway down the bore. Since it un- 
doubtedly made metallic contact with the steel it was allowed to re- 
main. Five bullets were driven through the other barrel also. To 
increase the amount of cupro-nickel present, filings from jackets 
were thrown into the bores. The whole was coated with oil A and 
exposed to 100 per cent humidity. No corrosion developed in either 
barrel. 

NO EVIDENCE OF GAS DIFFUSION. 

The observations recorded in Table 1, C and D, contradict Pas- 
dach's theory of diffusing acid gases.-^ 

Corrosion once initiated proceeded rapidly for a few days only, 
then ceased. Moreover, although rifles A, B, C, and D v/ere fired 
on the same date, June 6, two of them (C and D) ceased to corrode 
by June 11. According to the gas-diffusion theory, all the corrosive 
gas had therefore passed out of the steel. Kifles A and B were 
maintained at the same temperature, and it was therefore proper 
to believe that such gas in their barrels had diffused away, and that 
in consequence no after-corrosion would appear. When exposed to 
approximately 100 per cent humidity on June 16, however, both 
rifles corroded heavily. 

Further, the prevention of after-corrosion by the use of certain 
aqueous solutions, such as the saturated solution of sodium carbonate 
recommended by the Ordnance Department, does not accord with 
this theory, for it is difficult to conceive how such a solution could 
penetrate beneath the surface of the steel and so remove gases pre- 
viously dissolved or occluded there. 

Again, this gas theory fails to explain the pitting action of the 
after-corrosion. If the steel acts as a solvent, one may expect the 
attack of the diffusing acid gases to take place over the entire surface. 
Examination of corroded bore sections, however, shows centers of 
corrosion completely surrounded by bright metal, as illustrated by , 
Plate lY, A. 

Moreover, the contention that corrosive acid gases are produced 
by the explosive decomposition of the nitrocellulose under high pres- 

'■" See footnote 9 of this paper. 



10 COEROSIOKT UNDER OIL FILMS. 

sures is contrary to the testimony of a number of investigators of 
high standing.2^ 

Carbon dioxide is the only acid gas produced ; in the corrosion of 
iron and steel, this gas plays an unimportant part, or none at all. 2* 

The gases of explosion also contain hydrogen and carbon monoxide 
and as they have cooled from a very high temperature, the presence 
of oxides of nitrogen is impossible. According to the Ordnance 
Department, the mean temperature of the gases throughout the bore, 
during the explosion of the present service cartridge is about 
2,150° C. (3,900° F.).25 

Similarly, possible nitrogen oxides from the primer are eliminated. 
Among the probable gaseous products from the primer are hj^drogen 
sulphide from the sulphur, nitrogen and carbon monoxide from the 
mercury fulminate. An inorganic nitrate such as potassium nitrate 
gives the carbonate, nitrogen, and water. Potassium chlorate loses 
oxygen and gives the chloride, a neutral salt. 

ACID RESIDUES ON THE BORE SURFACE. 

Although the " acid gas " theory is thus eliminated, those theories 
that predicate a surface residue which may furnish acid are not so 
improba})le. For instance, partly burned nitrocellulose may be 
lodged on the bore, and its slow decomposition under atmospheric 
conditions can furnish very corrosive oxides of nitrogen. 

To ascertain whether such residues do exist, rifles were fouled b}- 
firing 40 rounds of service cartridges in each. The bores were then 
treated with ammonia, or with dilute potassium hydroxide. The 
solutions obtained were examined for nitrate and nitrite, using hydro- 
gen peroxide and the phenol-disulphonic acid method of Chamot, 
Pratt, and Reclfield.^^ Neither nitrate nor nitrite was found. The 
bores, however, failed to corrode under oil, showing that the corrosive 
residue had been removed. 

Another fouled rifle was tested for acid by treatment with am- 
monia, followed by evaporation on a water bath. The residue was 
Nesslerized. No fixed ammonia was found, thus proving the absence 
of any acid whose ammonium salt is stable under these conditions. 

-■■' Noble, Andrew, Researches on explosives, Phil. Trans. Royal Soc. London, vol. 205. 
1905, pp. 201-236, series A, part III ; Brynk, A., Properties of powders and their action in 
closed chambers and cannon, translated from the Russian by Bernadou, Washinsrton. 1904, 
p. 46 ; Bi-aunswig, H., Explosives, translated by C. E. Munroe and A. L. Kibler, New York, 
1012, p. 105 (from Sarrau, E., and Vielle, P., Mem. poudr. salp., t. 2, pp. 126. 337; 
1884-85). 

** Cushman, A. S., and Gardner, H. A., The corrosion of iron and steel New York, 1910, 
p. 38 ; Dunstan, W. R., .Towett. IT. A. D., and Colliding. E., The rusting of iron. Jour. 
Chem. Soc, vol. 87, 1005, pp. 1548-74. 

-" Noble, Andrew, work cited. 

-"Cliamot, E. M., Pratt, D. S., nnd RedfieUl, II. W.. A study of tlie phenolsulphonio ju'id 
method for the determination of nitrates in water (Fourth paper), A modliiod pho- 
nols.ulplionic acid method, Jour. Am. Chem. Soc, vol. 33, 1911, pp. 381-384. 



EXPERIMEI^TS MADE. 11 

As these aqueous alkaline solutions do not chemically combine 
with and remove acid, their valuable property lies in a physical sol- 
vent action. 

POTASSIUM CHLORIDE — CORROSIVE FOULING. 

Accordingly, the water-soluble fouling was examined. The only 
material found was the potassium chloride from the decomposition 
of the chlorate. 

A priming composition containing silver permanganate, lead sul- 
phocyanide, antimony sulphide, and trinitrotoluene was then loaded 
into a number of cartridges which were made in all other respects 
similar to the present service cartridge. Rifles were carefully 
cleaned and rinsed with water to remove any soluble inhibitor of 
corrosion, then fouled with these cartridges, using 10 rounds in one 
rifle and 16 in another. After firing, the bores were coated with oil, 
without disturbing unduly any of the adhering fouling. Exposed 
to a humidity of approximately 100 per cent for a week,- neither 
rifle corroded. 

Ten rounds in another rifle, using cartridges similar in all respects 
except for containing potassium chlorate instead of silver perman- 
ganate, gave fouling which caused heavy corrosion under oil. 

When a steel surface is brightened with emery, then coated with 
potassium chloride crystals, and subsequently with oil, and exposed 
to approximately 100 per cent humidity with a free access of oxygen, 
corrosion follows over night. It is, therefore, difficult to resist the 
conclusion that this salt is the cause of the after-corrosion that fol- 
lows the use of the present service cartridge. 

POSSIBLE OBJECTIONS TO THE POTASSIUM CHLORIDE THEORY. 

Proponents of other explanations ^' may, hoAvever, object that this 
salt should be removed by the mechanical polishing of the bright 
bore surface; that a coating of potassium chloride fails to explain 
the occurrence following repeated •cleanings over long intervals of 
time : that it fails to explain the light sweating Avhich sometimes 
follows the use of the sodium carbonate solution and that it fails to 
explain the severity of after-corrosion and its continuance under 
conditions such that unoiled steel surfaces do not undergo atmos- 
pheric corrosion. Further, it may be suggested that the wide use 
and many indorsements of nonaqueous nitro-solvents and gun oils 
controvert this explanation and that the after-corrosion experi- 
enced in the arid regions of Arizona and the Sahara does not accord 
with, the experimental findings reported above. 

2^ Ordnance Department, work cited, p. 44. 



12 CORROSIO]^ UNDER OIL FILMS. 

It ma}' be shown, however, that all of these confusing points are 
in no way at variance with the conclusion that potassium chloride 
is the sole corrosive fouling deposited by the explosion of the 
present service cartridges. 

FAILURE OF MECHAJSTICAL CLEANING PROCESSES. 

Plate IV, ^, is a photomicrograph showing the characteristic tool 
wounds and fissures on the bore surface of a new barrel. The photo- 
graph was taken near the muzzle and should therefore represent the 
best part of the bore surface. The depth and number of such fis- 
sures are no doubt much greater on old surfaces that have been sub- 
ject to neglect. 

Potassium chloride is deposited over the surface and lodged 
within the fissures. Mechanical swabbing from the breech fills the 
fissures and so renders the salt invisible to the inspection from the 
muzzle or breech. 

Plate IV, 5 is a weighty argument against those cleaning processes 
that endeavor to remove the corrosive fouling by abrasives.^* 

Plates IV, A and B^ were made by R. Thiessen and G. L. 
Henneman, of the Pittsburgh station of the Bureau of Mines. 

SWEATING AND THE OCCURRENCE AND RECURRENCE OF CORRO- 
SION OVER LONG PERIODS OF TIME. 

The puzzling " leaking-out " effects evidenced by the occurrence 
and recurrence of corrosion after cleaning and by " sweating " are 
quite as simply explained. 

The preceding experiments have shown that high humidities are 
necessary to after-corrosion. When the atmospheric conditions are 
such that these humidities are attained, water vapor " leaks " in to 
the corrosive fouling already on the surface and corrosion results. 
Subsequent mechanical swabbing, Avith possibly the use of an oil in 
which potassium chloride is not soluble, may remove a part of this 
salt and rust while the remainder is rubbed into the surface pits and 
fissures where it is invisible to inspection. Here it remains inactive 
until local atmospheric conditions are such that the requisite high 
humidity is again attained when the process is repeated. As the time 
interval involved between periods of high atmospheric humidity is 
indeterminate, it is not surprising that intervals of weeks or months 
may be involved. On the other hand, an extended period of high hu- 
midity may furnish enough water vapor to develop the nuiximum 
.corrosion and to dissolve completed the potassium chloride, so that 
subsequent swabbing would carry out most or all of the corrosive 

=•* See Ommundsen, H., and Robinson, K. II.. Kiflos and anununition and rifle sbootini 
New York, 1915, pp. 2S2-28.S. 



EXPERIMENTS MADE. Ig 

fouling Tvitli the rust. This explains the rapid cessation of corrosion 
in the experiments of Tables 1, C and D. 

" Sweating out *' has been attributed to the action of acid fouling 
sealed under metal fouling and therefore inaccessible to cleaning 
agents.-'^ This explanation can not. however, be correct, for it would 
likewise prevent water vapor from reaching this fouling. Such 
sealed-in fouling must therefore remain inactive. 0])viously. the 
fouling that causes *' sweating '* must be in communication with the 
outside atmosphere. 

As the chloride is driven into the fissures by enormous pressures,, 
and as it can be removed only by a suitable solvent whose action may 
be delayed by insoluble fouling — possibly, though not necessarily, 
metal fouling — at the mouth of the fissure, it is to be expected that a 
hasty swabbing with even an aqueous solution will fail to remove all 
the corrosive fouling from the more inaccessible pits. In that event, 
subsequent exposure to high humidities will be followed by a light 
corrosion or " sweating.'' The use of aqueous ammonium per^ 
sulphate solution for the removal of metal fouling also prolongs the 
solution process and so prevents " sweating out." The obvious, 
remedy is to prolong the cleaning process enough. The writer, 
since he has given attention to this point, has never encountered 
" sweating out " after the use of aqueous cleaning agents. 

AFTER-CORROSION AT HUMIDITIES BELOW SATURATION. 

The evidence so far presented shows that after-corrosion proceeds, 
at humidities of approximately 100 per cent but does not proceed at 
humidities of approximately 50 per cent. For the complete ex- 
planation of some of the phenomena of after-corrosion, however, it 
was necessary to determine the humidity relations somewhat more 
closely. 

Fifty service cartridges were fired in a rifle previously cleaned with 
sodium carbonate and ammonium persulphate solutions. The barrel 
was then sav^ed into sections and exposed under oil to the various 
humidities in atmospheres above saturated solutions of chemically 
pure salts in equilibrium with the solid phase. The temperatures 
were maintained constant at 30° C. by immersion in a thermostat. 
Every precaution was taken to prevent condensation. The salts and 
solution were placed in the thermostat a number of days before the 
sections were exposed, and immediately before exposure each section 
was warmed to a temperature higher than 30° C. 

Corrosion occurred above a saturated solution of sodium chloride 
but did not occur above a saturated solution of calcium nitrate. 

=® Ordnance Department, work cited, p. 44. 



14 CORROSION UNDER OIL FILMS. 

Parallel experiments, using sections of a rifle barrel thstt had never 
been fired and chemically pure potassium chloride furnished by Baker 
and Adamson gave identical results. 

Marshall gives the following values for humidities that may be 
obtained by such saturated solutions : 

i TPTTiTipr i Relative 



atiire 



ity 



° C. ■ 

Sodium chloride l 15 , 76 

Calcium nitrate I 18 ' 62 



These values are not exact and do not apply to temperatures other 
than those stated. The}^ do represent, however, the approximate 
humidities obtained, and because local temperature differences, vari- 
able impurities in the constituents of the explosives, and variable 
surface conditions of the bore all affect the exact humidity at which 
after-corrosion begins, highly accurate estimations would probably 
not enable one to predict possible humidity relations with very great 
accuracy. 

It is evident, therefore, that although after-corrosion requires 
high humidities, these humidities are well below the dew point (100 
per cent humidity). 

THE MECHANISM OF AETER-CORilOSION. 

Students of corrosion have generally recognized that liquid films 
are indispensable. On pure, brightly polished steel, water vapor 
does not form a liquid film until the dew point is reached.^^ Potas- 
sium chloride, however, deliquesces at humidities below saturation, 
and so forms a corrosive liquid film in direct contact with the metal 
beneath the oil film. 

The evidence cited indicates that after^corrosion proceeds at a 
humidity somewhat lower than that at which pure potassium chlo- 
ride deliquesces. However, this may be caused by traces of impuri- 
ties such as sodium chloride in the corrosive fouling. The humidity 
necessary for corrosion is also less when capillary surfaces are in- 
volved.^^ 

It was predicted that a more deliquescent chloride would cause 
corrosion at even lower humidities. Sections from an unfired barrel 
coated with calcium chloride, then oil A, corroded above the satu- 
rated solution of calcium nitrate. 

^ Diinstan, Jowett, and Gouldlng, Avork cited, especially p. 1554. 

Brown, A. Cruni, On the chemical processes involved in the rusting of iron : Jour. Iron 
and SI(>(>1 Inst., 188.S, pp. 129-131. 

Friend, .1. Newton, The corrosion of iron and steel. I>ondon, 1011, pp. 17-lS. 
»i Friend, J. Newton, work cited, pp. 9()-n7. 



PREVENTION OF AFTER-COEROSION. 15 

OXYGEN A CONTRIBUTING FACTOR. 

As potassium chloride solutions do not corrode iron and steel in 
the absence of oxygen/- this element is also an essential factor in 
after-corrosion. 

SEVERITY OF AFTER-COKROSION AND ITS CONTINUANCE UNDER 
CONDITIONS SUCH THAT UNOILED STEEL SURFACES ARE NOT 
CORRODED. 

As under atmospheric conditions the necessary humidity for the 
potassium chloride liquid film is reached more readily and main- 
tained much longer than the dew point at which cleaned iron and 
steel rusts, after-corrosion proceeds more rapidly than the ordinary 
atmospheric corrosion of cleaned steel surfaces. 

Alkali chlorides are popularly considered very corrosive, although 
experimental evidence has been presented to show that saturated 
solutions of such chlorides at all temperatures are less corrosive than 
pure water ,^^ and that at certain temperatures all concentrations are 
less corrosive.-^- The popular conception probably rests largely on 
the ability of the chlorides to form corrosive liquid films at humidi- 
ties below saturation. 

AFTER-CORROSION IN ARID REGIONS. 

Nothing here presented contradicts the testimony of riflemen who 
have encountered after-corrosion in the desert regions of Arizona or 
on the sands of the Sahara, for although such regions are arid and 
the humidities at noonday are relatively low, the coming of night 
is accompanied by a rapid cooling of the atmosphere. Hence, 
although the absolute water content of the atmosphere may not 
change materially, the drop in temperature may be, and often is, so 
great that precipitation of dew takes place. In certain desert 
regions such dew forms an important part of the water supply of 
the inhabitants. Special reports furnished the writer from stations 
of the United States Weather Bureau in the arid regions of Arizona, 
Utah, and Nevada showed that all of these stations have observed 
the formation of dew. 

POSSIBLE ABSENCE OF AFTER-CORROSION IN HIGH ATMOSPHERIC HUMIDI- 
TIES. 

As the humidity relations involved depend only on the tempera- 
ture of the bore surface and the absolute water content in the 

'2 Friend, J. Newton, work cited, p. 142. See also Adie, R., On the corrosion of metals : 
Minutes Proc. Inst. Civil Eng., vol. 4, 1845, pp. 323-331. 

33 Adie, R., On the corrosion of metals: Minutes Proc. Inst. Civil Eng-., vol. 4, 1845, 
pp. 323-331. 

^ Friend, J. N., work cited, especially pp. 139-142 ft". 



16 CORROSION UNDER OIL FILMS. 

atmosphere, a bore surface maintained at a temperature well abo^e- 
the atmospheric temperature will never rust. Thus the maximum 
Avater vapor possible in an atmosphere of 15° C. (59° F.) corre- 
sponds to a partial "pressure of 17.2 grams per sq. cm. If the bore 
surface is at 25° C. (77° F.), any water on it possesses a partial 
pressure of 32 grams per sq. cm. The maximum relative humidity 
possible at equilibrium under these conditions is less than 55 per- 
cent when referred to the temperature of the bore surface, conse- 
quently no after-corrosion can result. Here is an explanation of 
the decreased corrosion during winter and during storage in warm 
places indoors. 

PREVENTION OF AFTER-CORROSION. 

With the information now available it is possible to prescribe sim- 
ple methods for preventing after-corrosion. 

EXCLUSION OF WATER VAPOR. 

One method consists in excluding water vapor, which niay be 
done by tightly stopping both ends of the bore, as soon as convenient 
after firing. 

Greases and oils differ in their ability to transmit water vapor and 
oxygen but both do transmit them, and are in consequence unsuit- 
able for this purpose. The protective action of oils against ordi- 
nary atmospheric corrosion depends upon certain relations involving- 
the surface energies of the liquid water and oil. A preferential 
coating of the steel by the oil prevents the contact between the steel 
and the liquid water. 

SOLUTION AND REMOVAL OF POTASSIUM CHLORIDE. 

As already shown, the anticorrosive value of the saturated sodium 
carbonate solution and of the ammonium hydroxide solution de- 
pends upon the water content, for this dissolves the potassium 
chloride. 

When such solutions are not available, water alone may be used. 
It is best applied, when possible, by placing the muizle of the barrel 
in a receptacle containing water, preferably hot water. From the 
breech a rod and swab is used as a piston to pump the water thor- 
oughly over the entire bore surface for several minutes; then the 
bore should be treated further to remove cupro-nickel fouling, or, if 
preferred, it may be polished with dry swabs and oiled. 

It may be that the sodium carbonate and the ammonia solutions 
possess some value as detergents for the removal of insoluble foul- 
ing. Such cleansing properties can be supplied by soap, a reagent 
universall}^ available. 



PREVEXTIOX OF AFTER-CORROSION. 17 

Corrosion tests of rifles fouled with service cartridges, then cleaned 
with Trater alone, and with soap and water, showed that such treat- 
ment completely prevented after-corrosion. 

The statement by the Ordnance Department that the sodium car- 
bonate solution must be at least 20 per cent in strength ^^ is clearly 
not correct, at least in so far as it applies to the removal of the 
corrosive residue from the ball cartridges under consideration, and 
although there seems to be no objection to the continued use of this 
solution, it is not indispensable. 

NONAQUEOUS " NITROSOLVENTS " AND '' GUN OILS." 

Practical riflemen seem to have a deep-seated prejudice against 
the use of water and aqueous solutions in firearms. Other agent's, 
'nonaqueous, and variously denominated as "powder solvents," "ni- 
trosolvents," " rust removers," or " gun oils," are widely used. 

A number of these have been analyzed by X. A. C. Smith under 
the direction of E, W. Dean, of the petroleum division of the Bureau 
of Mines, and submitted by the writer to corrosion tests on fired 
rifles. The humidit}^ was approximately 100 per cent. The results 
follow : 

Results of tests of rum oUs. 
•Gun grease B : ■ 

Analysis: Crude petrolatum. 

Corrosion test: A coating of this grease did not prevent after-corrosion. 

3ust remoyer C : Per cent. 

Analysis: Refined petrolatum 51. 40 

Siliceous matter containing iron (probably tripoli) 48.25 

Amyl alcoliol . 35 

Coi^'osion test. — A coating of this rust remover did not prevent after- 
corrosion. 

Taste D: 

Analyst*: Percent. 

Dry soap 42. 85 

Mineral oil 43.60 

Water Trace. 

Amyl alcohol .: 13. 45 

Insoluble in alcohol and petroleum ether -. . 10 

Corrosion test: A coating of this paste .did not prevent after-corrosion. 
■Gun grease E : 

Analysis: Per cent. 

Tallow 97. 50 

Graphite 1. 94 

Oil of marjoram 0.56 

Corrosion test: A coating of this grease checked but did not completely 
prevent after-corrosion. The black of the graphite partly masks the 
brown of the rust, rendering difficult the ascertaining of the extent 
of the corrosion. 

^ Ordnance Department, work cited, p. 4.1. 



18 CORROSION UNDER OIL FILMS. 

Oils F and G: 

These oils are a mixture of animal and mineral oils practically identical 
with oil A and, like oil A, give no protection against after-corrosion. 
Nitrosolvents H and K: 

Analysis: Each is a mixture of a neutral mineral oil with amyl acetate.. 

The specific gravity of each at 15.5° C. is 0.859. 
Corrosion test. — These nitrosolvents 'neither removed the corrosive residue 
nor prevented after-corrosion. 

Oil L: 

Analysis: A pure, highly refined mineral oil whose specific gravity at 

15.5° C. is 0.853. 
Corrosion test: A coating of this oil did not prevent afuer-corrosion. 
Nitrosolvent M: 

Analysis: rer cent. 

Ammonium oleate 8. 03 

Free oleic acid 8.20 

Neutral saponifiable oil 21.50 

Mineral oil 34. 25 

Nitrobenzene 14. 50- 

Amyl acetate 1 13. 52 

Corrosion test: Characteristic sifter-corrosion did not develop under a 
coating of this mixture. After a day's exposure, swabbing gave a smudge 
resembling a mixture of iron and metal fouling oleates and left the bore 
bright, probably in part at least because of the presence of ammonium ole- 
ate, although the exact mechanism is not clear. The mixture forms an 
emulsion with water. It attacks the metal-fouling in the presence of 
air, forming green oleates, but is not an efficient solvent for metal foul- 
ing. 
Oil N: 

Analysis: I'ev cent. 

Mineral oil (apparently kerosene) . 84.4 

Neutral saponifiable oil 5.5 

Oleic acid 10. 1 

Corrosion test: Under a coating of this oil the steel bore was corroded. 
The product seemed to be nn iron oleate rather than the usual hydrate 
oxide of iron. The bore polished bright. 
Gun oil : 

Analys is : Per f-ent. 

Ammonium oleate 11. 3 

Oleic acid 11. 1 

Acetone 53. 4 

Mineral oil and amyl acetate 24.2 

Corrosion test: A coating of this gun oil functioned much like a coating- 
of nitro solvent M, developing on short exposure a characteristic green, 
and on extended exposure a mixture of oleates that were readily swabbed 
away. Contrasted with nitrosolvent M, this "oil" seemed to be the 
less desirable. 

Oil R: 

Analysis: Ter cent. 

Mineral oil 06. 8 

Volatile matter (acetone and amyl acetate) 23. G 

Neutral saponifiable oil 10.2 



PREVENTION OF AFTER-COREOSION. 19 

Pastes S and T: 

Analysis: ■ 

S T 

Per cent. Per cent. 

Petrolatum 36. 8 29. 6 

Sapouifiable oil 1.6 6.0 

Free acid 0.5 0.8 

Abrasive » 61. 1 63. 6 

Corrosimi tests: These pastes did not completely remove the corrosive 
residue nor prevent after-corrosion. 
Solvent V: 

Analysis: Percent. 

Mineral oil 80. 3 

Amyl alcohol 19. 7 

Corrosion test: This solvent neither removed the corrosive residue nor 
prevented after-corrosion. 

° The abrasive contained magnesium and aluminum, with a small quantity of iron and 
chromium. They were probably finely ground spinel. 

The number of failures in the corrosion tests is striking. Many 
of the mixtures are not onh^ inadequate but, because of the false 
sense of security that their use may induce, are even dangerous. If 
after the use of one such mixture the rifle is maintained at a humid- 
ity low enough, no corrosion will result. Therein appears to lie the 
true explanation for the supposed value of such a mixture and for 
much of the confusion that surrounds the problem of after-corrosion. 

OIL-WATER EMULSION. 

The superior qualities of the mixtures containing ammonium 
oleate, and the claims of Klever^® that an emulsifiable oil will dis- 
solve or remove the corrosive residue, suggested the theory that the 
protective action Avas due to the formation of an emulsion, the aque- 
ous phase of which dissolved the potassium chloride, while the non- 
aqueous phase or peptinizing agent prevented corrosion. 

To test the comparative corrosive properties of water, of water- 
annnonium oleate. and of water- ammonium oleate-oil systems, six 
plates 2.5 by 5 by 0.2 cm. were cut from the same piece of steel and 
machined to a uniform surface. To facilitate suspension a 0.5 cm. 
hole was drilled through each. 

All the plates were then carefully washed successively with soap 
and water, a hot 10 per cent sodium-carbonate solution, distilled 
water, alcohol, and, finally, ether. Two were suspended in distilled 
water, two in a mixture of distilled water, 100 parts, with ammonium 
oleate 2 parts, and two in an emulsion of one volume of the above 
ammonium oleate mixture with one volume of a neutral transformer 
oil (specific gravity 0.855 at 15.5° C). The depth of immersion 
and the access of light and of oxygen were indentical in all three 
systems. The results are shown in Table 2. 

^ Klpver. Fredrjch W.. work cited. 



20 



CORROSION UNDER OIL FILMS. 



Table 2. — Relative corrosion in water, in aminonium oleate plus loater, and in 
an oil-water emulsion, containing ammonium oleate. 



First exposure, 2 days. 



Plate 

No. 



Medium. 



Ammonium oleate plus water. 

do 

Water-oil emulsion 

do 

Water 

do 



Gain (+) or 
loss (— ) in 
weight, mg. 



+ 0.5 
- .1 
+ .1 
+ .2 
-12.5 
-12.5 



Second exposure, 5 days. 



Plate 

No. 



Medium. 



Gam (+) or 
loss (— ) in 
weight, mg. 



Ammonium oleate plus water. . ! No change. 

..-.do ! - 0.1 

Oil-water emulsion j — .3 

do ! No change. 

Water i -28. 3 

....do I -27.1 



Through both exposures, plates 1, 2, 3, and 4 maintained their ini- 
tial polish. After the second exposure each of the six plates was im- 
mersed in its solution for an hour, then withdrawn and exposed to a 
laborator}^ atmosphere for 27 days. 

At the end of that time the change in weight was determined. 



Table 



-^Relative corrosion in air after immorsion in various media. 



Plate 

No. 


• Medium with which the plate was initially coated. 


Gain (-I-) 

or loss ( — ) 

in weight, 

mg. 


1 


Ammonium oleate and water . . 


1 -2.8 


2 


do 


! -2.5 


3 




-0.3 


4 


do 


i -0.4 


5 


Distilled water 


i -0.1 


6 


do 


\ -0.5 







The loss in weight of plates 1 and 2 was due to the formation of 
iron oleate, which could be easily seen on the surface of the plate. 
It was readily removed, leaving the initial polish somewhat dimmed, 
although the change was too small to be detected under the unfavor- 
able conditions that prevail on a bore surface. 

The slight loss in weight of plates 3 and 4 was probably due to the 
formation of iron oleate. 

The anticorrosive properties of the ammonium soap solution is 
seemingly not unusual, for sodium and potassium soaps in distilled 
water likewise prevent corrosion even in the presence of an excess 
of oleic acid. In the presence of certain concentrations of potassium 
chloride this anticorrosive property may be diminished or comj^jletely 
lost, although this point was not specially investigated. Thus, a 
piece of steel coated with an aqueous solution of potassium chloride 
(1 i)er cent) containing a sodium soap (about 2 per cent), and then 
exi)osed, rusted much as a rifle bore rusts uucUm- an oil lihn. A second 
piece coated only with distilled water containing about 2 per cent 



PREVENTIOiSr OF AFTER-CORROSION. 21 

of the same soap remained perfectly bright under analogous condi- 
tions. 

The protective action of the soaps can hardly be due to their 
alkalinity, for the ammonium soap solution retained its noncorrosive 
properties after air had been blown through it for several days to 
drive out any free ammonia that might have been formed by hy- 
drolysis. 

Fouled rifle barrels were coated with an emulsion of ammonium 
oleate, oil, and water, and exposed to a high humidity. Similar ex- 
periments were made using a commercial emulsifiable oil which 
formed an emulsion when exposed to high humidities. The pro- 
tection obtained was in general that predicted from the above tests. 
Cloth swabs left the bore bright, carrying out oleates of the metal- 
fouling and iron and occasionally a pinhead of rust. Although 
superior to neutral animal and mineral oils when used alone, and to 
most of the nonaqueous agents tested, the emulsion and emulsifiable 
oil did not afford the perfect protection given by a thorough treat- 
ment with water followed by a coating of neutral animal or min- 
eral oil. 

In many rifles that do not have a bolt action the use of water or an 
aqueous alkaline solution entails the danger unless the barrel is dis- 
mounted, of introducing such agents into the breech mechanism. 
For such rifles the solution of the potassium chloride by a suitable 
oil- water emulsion is preferable as this emulsion is noncorrosive and 
its evaporation leaves an oily residue that will not ruin the breech 
mechanism. 

Soon after the application of this emulsion all surfaces with which 
it has made contact should be swabbed clean and coated with a 
suitable oil: this precaution will prevent undesirable effects follow- 
ing the use of a property compounded emulsion. 

A TEST FOR USEFUL SOLVENTS. 

It is often desirable to determine without exposing a valuable fire- 
arm to possible corrosion whether a solvent is useful or not. For 
this purpose a soft steel plate may be brightened with emery paper 
and rubbed with potassium chloride. The oil or solvent should then 
be used as directed, with special care not to dislodge mechanically 
the salt from direct contact with the steel. The plate should then 
be exposed to a saturated humidity for not less than a week. Only 
those processes that give complete protection should be employed on 
the bore surfaces of the firearm. 



22 coRROSioisr under oil films. 

ADDITION OF AN INHIBITOR OF CORROSION. 
BLACK POWDER RESIDUES. 

The various patented alkaline greases and pastes designed to be 
left in the gun barrel owe their value not to the neutralization of 
acid, but to the property, which many alkalies have, of inhibiting 
corrosion. The noncorrosive property said to be possessed by black 
powder residues ^^ seems to depend upon the large percentage of 
potassium carbonate formed by the decomposition of the nitrate. 

Any reader interested in the composition of the residues obtained 
from black powder will find in Guttmann,^^ a discussion of this sub- 
ject and many references to the original literature. The black 
powder residues are largely water soluble and may be readily re- 
moved by a suitable aqueous medium. 

The writer has prepared a simple inhibiting paste b}^ gelatinizing 
a strong solution of sodium carbonate with soap. This paste may be 
used for a preliminary protection in the field. The best results are 
obtained when the first application is immediately removed with 
water and a second coating applied. It is an excellent detergent, pos- 
sesses many of the properties of the sodium carbonate solution and 
may be readily transported and handled. 

NONCORROSIVE PRIMING COMPOSITIONS. 

The simplest remedy for after-corrosion consists in using car- 
tridges that do not deposit a corrosive fouling, that is, cartridges 
containing only ingredients whose explosive decomposition yields 
either a residue insoluble in water or a residue whose solution in 
water does not corrode steel. In practice, only compounds containing 
halogen must be avoided, and consequently the elimination of only 
one ingredient, the chlorate of the primer is involved, yet the problem 
is of no mean magnitude. The priming composition is one of the 
most vital of the cartridge parts, and the universal use of composi- 
tions containing potassium chlorate has survived many years of 
study and experience. 

The substitute oxidizing salt should be one whose admixture with 
a suitable reducer is very sensitive to percussion, stable under ex- 
tremes of atmospheric temperature and moisture for long periods of 
time, and not more hygroscopic than potassium chlorate. Preferably, 
it should be endothermic and contain a large percentage of oxygen, 
that the energy developed by the decomposition may bo high. 

8' Marshall, Arthur, Explosives, their manufacture, properties, tests, and history. Lon- 
don, 1915 ed., p. 61. 

88 Guttmann, O., work cited. 



PREVENTION OF AFTER-CORROSION. 23 

No halogen- free salt at present available seems to satisfy these 
requirements, as nonchlorate primers in g-eneral possess less power, 
less stability, or less sensiti\dty than chlorate primers. Perhaps the 
most satisfactory substitute is barium nitrate. Permanganates of 
potassium and silver are more sensitive, but much less stable. Most 
of the remaining oxidizing salts are of little value. 

Mixtures of a nitrate, such as barium nitrate, with antimony sul- 
phide or lead sulphocyanide are rather insensitive to percussion, and 
it is not feasible to prepare primers of the " burning type " based on 
such mixtures. Mixtures containing barium nitrate and red phos- 
phorus are much more sensitive and are also noncorrosive. Success 
in using such a priming composition will probably depend largely 
on the behavior of the red phosphorus, whose stability under storage 
is yet to be demonstrated. Red phosphorus primers have been 
patented ^^ and were used by Germany during the war.*° 

Noncorrosive primers that do not contain red phosphorus may be 
jriven the requisite sensitivity by the introduction of a suitable de- 
tonating substance, such as mercury fulminate. Primers of the ful- 
minate, nitrate, and antimony sulphide type, are, however, less pow- 
erful than the corresponding chlorate mixtures. To remedy this, it 
is necessary to increase the percentage of fulminate according to the 
suggestion of Mertens *^ or to add a reenforcing explosive compound. 
The writer prefers the latter method. A nitro body very sensitive 
to percussion seems to be the logical re-enforcer. Such a nitro body 
is lead picrate. Trinitrotoluene is too insensitive. 

Preliminary experiments on a number of noncorrosive mixtures 
showed the following formula to be very promising : 

Formula for a noncorrosive mixture. 

Per cent. 

Mercury fulminate 15 

T^ead picrate 25 

Antimony sulphide 20 

Barium nitrate (impregnated with 1 per cent diphenylamine) 40 

To date, however, the experimental adaptation of such a composi- 
tion to the pyro powder used in the service has not been completed. 

The advantages of noncorrosive primers and the principles govern- 
ing this composition now seem quite plain. General technical appli- 
cation should follow. 

«» German Patent 274,000, Klasse 78e, Gruppe 2, Deutsche Waffen-und Munitions- 
fabriken in Karlsrulie 1. B., Verhahren zur HersteUung von phosphorhaltigen Zundsatzen, 
November 29, 1912. 

*o Chemical Abstracts, New detonators made without fulminate of mercury, vol. 13, 191 9, 
p. 263. From Prof. Notes, Proc. U. S. Naval Inst., vol. 44, 1918, p. 2625. 

^^ Mertens, Otto, work cited. 



24 CORROSION UNDER OIL FILMS. 

AFTER-CORBOSION FROM LOW-PRESSURE NITROCELLULOSE 

POWDERS. 

Initially, the scope of the investigations described was limited to 
the study of the corrosion that follows the use of the present high- 
pressure service cartridge and the foregoing discussion has dealt 
only with this corrosion. 

With cartridges generating lower pressures, the incomplete com- 
bustion of the nitrocellulose to corrosive acid products is much more 
probable. Although it was not possible to make a complete study 
of this point, a few tests using noncorrosive priming compositions 
and a low-pressure nitrocellulose have shown that corrosion from 
the nitrocellulose powder occurs only with extremely low confining 
pressures, such as those of blank cartridges. No nitrocellulose cor- 
rosion followed the use of a low-pressure gallery powder confined 
behind the full-weight .30-caliber bullet. 

THE GENERAL PROBLEM OF CORROSION UNDER OIL FILMS. 

It is believed that the foregoing studies give information applica- 
ble to many other examples of corrosion under oil films. Thus, a 
water-soluble salt (sodium chloride) is an important part of per- 
spiration residues, and such residues may be expected to cause corro- 
sion even under oil films at humidities below those at which clean 
iron corrodes. Experience has shown that this actually happens. 
Perspiring depends on physiological conditions that A^ary among dif- 
ferent workers, thus explaining why bright steel surfaces packed 
beneath petrolatum b}^ one workman fail more frequently and cor- 
rode more frequently than similar surfaces packed by another. Rifle 
parts handled by persons afRicted with gout are said to corrode in 
spite of frequent oiling. 

A vast number of operations involve the handling of bright steel 
surfaces and the concomitant danger of salty perspiration residues. 
Moreover, a similar form of corrosion may be expected where iron 
or steel surfaces are exposed to finely divided corrosive salts car- 
ried mechanically through the air (a common condition near the 
ocean), or where such surfaces are exposed to certain corrosive vola- 
tile salts. 

Were all the surfaces upon Avhich such salts deposit accessible the 
prevention would be relatively simple. Such surfaces can be given 
a preliminary Avashing with Avater, followed b}' absorption of the 
water on cloths or other suitable porous agents. They may then 
be oiled with safety. Drying by evaporation is not to be recom- 
mended, as the water may contain soluble salts that would be de- 
posited. 



SUMMARY. 25 

Where the surfaces are inaccessible, the safest procedure is to fol- 
low the water wash with a suitable emulsion capable of absorbing 
water. This enuilsion should be removed as completely as possible; 
then the surface may be coated with oil. ^ 

SUMMARY. 

A review of the scientific, patent, and trade literature, and the 
compilation of the experiences of many practical riflemen has shown 
much confusion in the theories involved in the after-corrosion of 
firearms and much divergence in the practices recommended for pre- 
vention. Generally this corrosion has been attributed to acid prod- 
ucts on the bore, although other explanations, including the action 
of chlorides, has been advanced. A critical laboratory study, com- 
prising the exposure of fired rifles and fouled barrel sections to 
known humidities, the chemical examination of the corrosive res- 
idue, the use of special ammunition, and the analysis and testing 
of many '" nitrosolvents " and other compositions recommended as 
preventives showed : 

The present high-pressure smokeless cartridge leaves no nitrocellu- 
lose residue and no corrosive acid residue. After-corrosion follow- 
ing the use of such cartridges is caused by (1) the explosive deposi- 
tion of a water-soluble salt or salts in whose aqueous solution the steel 
corrodes, together with subsequent exposure to (2) a high humidity 
and (3) the presence of oxygen. In the present service ammunition 
this salt is potassium chloride from the decomposition of the chlorate 
in the primer. 

Such water-soluble salt or salts are retained in tool wounds and 
pits on the bore surfaces, in which they can not be seen and from 
which they can not be removed mechanically. They are easily dis- 
solved by water or suitable aqueous solutions. 

After-corrosion may also be prevented by keeping both ends of 
the bore tightly corked. 

The present service ammunition can be rendered noncorrosive by 
eliminating the chlorate of the primer. It may be possible to de- 
velop a noncorrosive primer that will not affect the present ballistic 
properties of the cartridge. This point is under investigation. 

A number of nonaqueous compositions recommended for cleaning 
firearms possess little or no value. Their supposed virtue seems to 
rest on tests conducted at humidities so low that no corrosion could 
occur. 

After-corrosion proceeds below the dew point. 

A simple test for differentiating between worthless and valuable 
cleaning compounds is described. 



26 (CORROSION UNDEE OIL FILMS. 

Corrosion from nitrocellulose powder occurs only when the con- 
fining pressure is extremely low, as in blank cartridges. 

The study has shown that after-corrosion is closely allied to a 
number of other problems of corrosion beneath oil films, and has 
indicated a simple method for eliminating the attending menace to 
many important iron and steel products other than firearms. 

ACKNOWLEDGMENTS. 

The writer Avishes to acknowledge the assistance of A. C. Fieldner^ 
supervising chemist, and E. W. Dean, petroleum chemist, of the 
Pittsburgh station of the Bureau of Mines, and of Dr. G. A. Hulett, 
Dr. C. E. Munroe, and Maj. Gen. F. C. Ainsworth, United States 
Army (retired). 

PUBLICATIONS ON THE CORROSION OF IRON AND STEEL. 

A limited suppl}^ of the following publications of the Bureau of 
Mines is available for free distribution. Requests for copies should, 
be addressed to the Director, Bureau of Mines. 

Technical Paper 15. An electrolytic method of preventing corrosion of iron 
or steel, by J. K. Clement and L. V. Walker. 1913. 19 pp. 

Technical Paper 236. Abatement of corrosion in central heating systems, by 
F. N. Speller. 1919. 12 pp. 



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